Loosely lock-stepped processors are processors executing duplicate copies of a user program, but the processors need neither execute their respective copies of the user program in a step-for-step fashion nor at the same wall clock time. Loosely lock-stepped processors may be used to detect and correct computational faults, such as caused by cosmic radiation, and also may be used to ensure the user program continues to operate in spite of a hardware failure.
Because of slight differences in clocking frequency, or correctable soft errors experienced in one processor but not others, one processor may lead in its execution point within the user program, and it is difficult to determine whether a user program in a processor leads or lags. A comparison of program counters or instruction pointers may not adequately identify a user program's execution point inasmuch all the user programs may be in the same software loop, but at different iterations of the loop, and yet the instruction pointer may be the same. Further, some processors implement retired instruction counters in their performance monitoring unit that attempt to increment on each retired instruction. A comparison of retired instruction counter values may be insufficient inasmuch as retired instruction counters are included for performance monitoring purposes, and do not necessarily correctly count each and every retired instruction. For example, some processors have the ability to cease loading of new instructions once the retired instruction counter reaches a particular value; however, a pipelined processor may have several instructions still in the pipeline that complete and that are not counted by the retired instruction counter, known as a sliding stop. Moreover, processor manufacturers do not guarantee that their retired instruction counters are one hundred percent accurate.